Drilling of a wellbore may be enhanced by generating pressure pulses in a drilling fluid flowing through a drill string as the wellbore is being drilled. The pressure pulses may vibrate the drill string to reduce frictional forces between the drill string and the wellbore, and may induce a percussive effect at the drill bit to help advance the drill string through the wellbore.
The following prior art references disclose a variety of downhole apparatuses for generating pressure pulses in a drilling fluid flowing through a drill string: U.S. Pat. No. 3,958,217 (Spinnler); U.S. Pat. No. 5,040,155 (Feld); U.S. Pat. No. 6,053,261 (Walter); U.S. Pat. No. 6,484,817 (Innes); U.S. Pat. No. 6,508,317 (Eddison et al.); U.S. Patent Application Publication No. 2012/0048619 (Seutter et al.); and U.S. Patent Application Publication No. 2012/160,476 (Bakken).
In particular, U.S. Pat. No. 3,958,217 (Spinnler) discloses a mud pulse telemetry system for transmitting information from the bottom of a well hole to the surface. The mud pulse telemeter includes a pulsing valve operated by a pilot valve mechanism, which is in turn controlled by electrical input power. U.S. Pat. No. 5,040,155 (Feld) discloses a double guided mud pulse valve that includes a main valve body and an auxiliary valve. To produce a pressure pulse in a drilling medium, the auxiliary valve is controlled by a device for determining drilling measurement data. U.S. Pat. No. 6,508,317 (Eddison et al.) discloses a downhole flow pulsing apparatus that includes a valve located in the throughbore of a housing. A fluid actuated positive displacement motor is associated with a movable valve member to vary the area of the valve's flow passage, and provide a varying fluid flow therethrough. The apparatus may be provided in combination with a drill bit and a pressure responsive device that expands or retracts in response to the varying drilling pressure created by the varying flow passage area, to provide a percussive effect at the drill bit.
There remains a need for a downhole apparatus for generating pressure pulses in a drilling fluid flowing through a drill string, and more particularly such an apparatus that is suitable for inducing a percussive effect at a drill bit.